In semiconductor production processes, wafers are handled by the so-called cassette-to-cassette system wherein wafers are transported, as placed in wafer baskets or carriers, from step to step. This system has achieved remarkable increases in the quantity of semiconductors produced. The wafer production process is practiced entirely within a clean room or clean bench which is diminished in the amount of dust present to keep the surface of semiconductor wafers clean at all times.
Conventional wafer baskets are left open at opposite sides and at its upper portion for ease of cleaning, drying, transport or transfer, while wafers are generally left exposed when handled with carriers. Accordingly, wafers are very likely to be contaminated with small dust particles or the like remaining in the clean room. Wafer baskets are therefore transported or stored usually as accommodated in a closed box, whereas not infrequently, the wafer basket is transported as it is, or wafers are left exposed when transported with use of carriers, without considering the possibility of contamination. The influence of small dust particles is negligible when the width of patterns to be formed on semiconductor devices is small, but even dust particles as small as about 0.1 .mu.m produce an adverse influence as the pattern width decreases with an increase in the complexity of integration as from LSI to VLSI. Thus, it has become increasingly important to prevent dust deposition on the wafer surface.
Conventional carriers, wafer baskets for cleaning, baskets for drying, baskets for transport and storage cases are predominantly synthetic resin moldings, and most of them are electrically chargeable insulating moldings which are prepared by molding polypropylene (PP) or like resin as it is (without reinforcement). Presently, therefore carriers, baskets and storage cases are damaged by wafers, producing dust, becoming charged to attract dust and permitting promoted deposition of dust on wafers to result in a high reject ratio. Further since the resin used is not reinforced, the baskets for cleaning or drying have the drawback of having insufficient thermal rigidity. On the other hand, in the case where carriers, baskets, storage cases and like wafer holding molded articles are reinforced with glass fiber, carbon fiber, chopped fiber or the like, it is impossible to internally reinforce the moldings up to the outermost surface thereof when observed microscopically. Consequently, it is always likely that the moldings will be damaged by wafers, or wafers themselves will be damaged by moldings which are as high as 6 to 7 in Moh's hardness.
On the other hand, the resin material is subjected to an antistatic treatment usually by adding carbon black thereto, whereas carbon black is extremely fine, very low in compatibility with the resin and prone to become released and scattered about to contaminate wafers owing to abrasion or frictional contact with wafers. Although investigations were conducted on surfactants for use as antistatic agents, they fail to retain a sustained antistatic effect and become a dust attractant like insulating moldings. Moreover, even the presence of a very small amount of vapor of surfactant could lead to the serious drawback of fogging up the wafer surface.
Further in the case of liquid crystal panels, rapid progress has been made in providing precision display screens with use of glass substrates of reduced thickness, which need to fulfill the same requirements as semiconductor wafers.
We have conducted intensive research to overcome the foregoing drawbacks of the prior art and consequently found that the contamination of wafers is attributable largely to incorporation of air in the cleaning step and to charging of air in the drying step. More specifically, when wafers as contained in a basket are dipped in a cleaning solution (acid, alkali or the like) for cleaning, many air bubbles adhere to the surface of the wafers, which in turn are contaminated with dust present in the bubbles. Since wafers are dried in air having a temperature of about 200.degree. C., the air becomes charged when flowing rapidly, consequently charging the basket, which in turn attracts dust or the like to contaminate the wafer.
We have also found that the contamination of wafers can be remarkably prevented over the entire production process if it is possible to preclude wafer contamination in these cleaning and drying steps.
An object of the present invention is to provide a molded article for holding wafers which is capable of preventing deposition of dust and extraneous matter on the wafers during cleaning, drying, transport or storage, especially during cleaning and drying, and which is less likely to be damaged by the wafer and to cause damage to the wafer itself.
Another object of the invention is to provide a molded article for holding wafers which has improved thermal strength (inclusive of rigidity) as required of cleaning and drying baskets.
Another object of the invention is to provide a molded article for holding wafers which is also usable for cleaning, drying, transporting or storing glass plates for liquid crystal televisions.
Still another object of the invention is to provide the above-mentioned wafer holding articles at a low cost without necessitating special handling.
The foregoing objects of the present invention, especially prevention of contamination of wafers in the cleaning and drying steps, can be accomplished by using a molded article (basket or the like) made of a thermoplastic resin containing whiskers.